This invention relates to a method and apparatus for detection and demodulation of binary phase modulated signals, and particularly to detection and demodulation of relative phase modulated binary data on a carrier.
Phase modulation of binary data on a carrier wave (electromagnetic or acoustic) is commonly used since the phase-shift of the carrier wave from one bit period to another is relatively easy to demodulate. In some systems the carrier is actually a subcarrier modulated on a steady-phase carrier. In either case, the modulated signal is tracked by the receiver in a phase-lock loop used to generate a reference signal for demodulation purposes (i.e., for phase detection). However, that and other techniques for extracting a phase reference from the signal received is normally exploited only in electromagnetic transmissions due to limitations of transducers employed in acoustic wave transmissions.
In systems which do not somehow transmit a phase reference, relative phase modulation is employed to encode the binary data on the carrier. For example, each bit period may consist of a precise number of carrier cycles, such as seven, with a phase reversal of one bit period to indicate a binary 0. Transmission of either phase (0.degree. or 180.degree.) may be selected for an initial bit period. If a 0 is the first data bit, the phase of the carrier is switched (inverted) at the end of that initial bit period if the next data bit is a binary 1, otherwise successive bit periods of switched phase represent a data bit 0. The convention could, of course, be opposite such that reversing the phase of successive bit periods represents binary 1 data bits, and not reversing the phase represents binary 0 data bits.
To demodulate relative phase modulated signals, the carrier received during each bit period is stored and compared with the phase of the carrier during the next period. Each period thus provides a reference phase for the next period.
A problem with relative phase modulation is demodulation in the presence of Doppler shift due to relative motion between the transmitter and receiver, particularly when the Doppler shift is not known in advance and cannot be independently determined.
To understand this problem which arises because of Doppler shift, assume a binary bit period of the modulated carrier to consist of seven cycles of the carrier. With a positive Doppler shift, the period of the seven cycles decreases while for a negative Doppler shift the period increases. Consequently, if an attempt is made to demodulate by comparing the phase of one bit period with the phase of a succeeding bit period, an error will occur because one bit period being compared with the other will be overlapping with another bit period, and this error would be cumulative such that after demodulating a 10 to 20 bit word, the demodulation of the last few bits would be totally unreliable. The error is, of course, the offset in the comparison of cycles in one data bit period with cycles of a succeeding data bit period. But even assuming that somehow demodulation has been properly effected in the presence of significant Doppler phase shift, there is still a problem in proper identification of the bits in the resulting (demodulated) signal.
To appreciate this last problem of identifying a properly demodulated signal in the presence of Doppler shift, consider trying to decode the first N bits of coded transmission by comparison with an N bit coded word stored at the receiver. It is common practice to transmit such a coded word for such comparison in order to determine when the first bit of a following message occurs, or to simply discriminate against noise where only the coded word is transmitted, as when a coded word is used to cause a receiver to respond in some way. An example might be to shut off a valve in a blowout prevention system for offshore drilling platforms. Serial or parallel comparison of the coded signal received with a stored replica would be impossible, unless the replica is somehow compressed or expanded by an amount approximately equal to the amount the data bit periods of the coded signal have been compressed or expanded due to any Doppler shift caused by relative motion between the transmitter and the receiver.